| Magnesium and its alloys have advantageous properties including high specific strength, high specific toughness, low elastic module, excellent damping characteristics, good machinability and dimensional stability, electromagnetic shielding, abundant in resource as well as easy to be recycled. These properties make magnesium and its alloys valuable in a number of applications such as automobile, electronics, 3C products, aircraft and astronautics, sporting goods, handheld tools and household equipment, and et al. However, poor corrosion resistance and wear resistance have restricted much wider uses of magnesium alloys. A suitable surface treatment is commonly needed to meet the requirements of various applications.There are many surface techniques adopted to produce a protective coating on magnesium alloys. This paper analyzed the properties of magnesium in detail, and investigated various surface techniques that is commonly adopted and discussed where the advantages and disadvantages are in each process. Then we chose arc-glow plasma penetrating technique to deposit Ti and Cr mono-layer, as well as Ti-Cr multi-layer coatings on magnesium alloy AZ91D. The aim of our study is to improve the corrosion resistance of magnesium alloys, support it much wider uses.Arc-glow plasma penetrating is a novel surface coating technique that has been developed by the Research Institute of Surface Engineering of Taiyuan University of Technology. Based on Double Glow Plasma Surface Alloying, one or more cold cathodes are introduced as arc sources in this process. Before coating, glow discharge heating and cleaning the substrates. Then enhanced by assistant glow plasma discharge, the source continually emits ion beams of coating elements with high current density and high ionizing ratio. As the ion bombard working on, the surface of the substrates has formed deposited layer, penetrated layer and hybrid layer. The coated films may be either metallic or ceramic. The coatings are generally dense, homogeneous and robustly adhered with matrix.In the process study, the effects of main parameters, e.g., temperature, working time, distance between specimens and arc sources, pressure, bias voltage, as well as arc current, on quality of the films were surveyed. Then the optimum process sets were concluded as thereafter: the temperature of substrates is 170～190℃, working time is 60～70min, distance between specimens and arc sources is 170～190mm, pressure alters is 0.5Pa, bias voltage×duty cycle alters from 650V×0.6 to 80V×0.8, arc current is 60～70A. The characteristics and properties of this mono-layer and multi-layer were tested and evaluated, which can be summarized as below:1. The surface appearances of the coatings are shinning and smooth, with uniform and attractive colors. The deposited films are dense and homogeneous, and the grains of Ti and Cr are fine. The thicknesses of the coatings are all above 1.5μm.2. The adhesion strength between coatings and substrate had all been evaluated by ways of both scratching and peeling. In the scratching tests, the scars on the three films were much smooth and no sonic signal had been recorded. Peeling tests were according to ASTM D3359-78. The adhesion strength grade of Ti film was 5B, 4B for that of Cr, and 5B for That of Ti-Cr multi-layer film. The adhesion strength of all films all had been qualified.3. Compared to the original hardness HV70 of matrix, all the coatings have increased the surface hardness of AZ91D significantly. That of the specimen with Ti coating increased 6 times, that of the specimen with Cr coating increased 3 times, and that of the specimen with Ti-Cr multi-layer coating increased 5 times. The increase of hardness is surely beneficial to the improvement of wear resistance of AZ91D.4. The results of composition distribution analyses by glow discharge spectrum (GDS) indicate: Between AZ91D matrix and Ti film, Cr film, as well as Ti-Cr multi-layer film, there are diffusion layers with composition changed in gradient. The gradual changes of composition reflected that alloying bonds had formed with Ti or/and Cr diffused in to matrix. The existence of diffusion layer surely reinforced the adhesion strength between coatings and substrates.5. In Cr film and Cr-Ti multi-layer film, there were new intermetallic compounds that XRD analyses identified as Al8Cr5, Cr5Al8, AlTi2 and Cr2 Ti. That is to say, chemical bonds existed between Cr film /Ti-Cr multi-layer film and substrate as well as between the sub-layer of Ti-Cr multi-layer film. Those chemical bonds firmly contributed a robust adhesion of the two films.6. The corrosion resistance of AZ91D with different coatings was evaluated by tests of salt spray, electro-chemical polarization, and exposure in outdoor atmosphere.(1) The test of salt spray showed: Just 4 hours later, large areas of erosion had taken place on the surface of AZ91D matrix. Comparing to that, etching pits appeared on Cr film when testing time reached 28 hours, and 48 hours for Ti and Ti-Cr films. After 48 hours test, the corrosion ratios of matrix, Ti film, Cr film, as well as Ti-Cr multi-layer film were 1.867%, 0.198%, 0.403% and 0.203% respectively.(2) The test of electro-chemical polarization showed: The corrosion potentials of AZ91D matrix and specimens coated with Ti, Cr, Ti-Cr films were -1500mV, -300mV, -900mV and -120mV respectively; the corrosion currents were 4.365mA/cm2, 0.007mA/cm2, 0.060mA/cm2 and 0.006mA/cm2 respectively; the corrosion rates were 19.78g/m2h, 0.031g/m2h, 0.358g/m2h and 0.037g/m2h respectively.(3) The test of exposure in outdoor atmosphere indicated: The corrosion rates of all specimens decreased with exposure time increased. However, the corrosion rate of AZ91D matrix is much larger than that of the specimens coated with Ti, Cr, Ti-Cr films. After 90 days of exposure, the specimen coated with Ti-Cr multi-layer film showed the lowest corrosion rate.It is obvious that all the three coatings have significantly improved the corrosion resistance of magnesium alloy AZ91D.The anti-corrosion performances of Ti film and Ti-Cr multi-layer film are better than that of Cr film. Multi-layer coating has advantages over mono-layer coatings in words of corrosion resistance, as experiments results illustrate that the corrosion rates of specimen with 1.523μm Ti-Cr film is even lower than that of specimen with 3.932μm Ti film.The corrosion mechanism of the magnesium alloy matrix as well as specimens coated with Ti, Cr and Ti-Cr films had been investigated. Not only in the salt spraying but also in the electro-chemical experiments, the corrosion behaviors of all specimens exhibited features of inter-grains corrosion. Inter-grains corrosion directly took place on the surface of matrix, but developed from etching pits on the surfaces of three kinds of films. Etching pits formed in the places where defects accumulated in the films. The time that forming an etching pit required is depend on the thickness and adhesion condition of the coatings. For all specimens, the corrosion process that took place during exposure test in outdoor atmosphere is also electro-chemical mechanism. Anode reaction is controlling step of corrosion in atmosphere.Finally, by the method of lattices gas automata, a model of grain formation and growth when depositing is going on was successfully set up. Based on the computer calculation and accordance of film growing mechanism, the author explained theoretically why and how the morphologies of films formed. Those 2 dimensional and 3 dimensional morphologies were observed in SEM and AFM photos respectively. Analyses illustrate: The films grow horizontally and grains are in branch geometry. However, if the initial nuclei are more enough, the film will grows much densely, and no distinct branch can be observed. Furthermore, if temperature, substrate and other conditions of growth changed, the branch geometry of grains may translate to a certain ordered shapes. The films grow in laminating; every layer grains grow according to island-model.Through systemic research, we can give out the conclusions of this study as below: Arc-glow plasma penetrating is an effective and practical process to protect magnesium alloys from corrosion. Three metallic coatings, Ti mono-layer film, Cr mono-layer film and Ti-Cr multi-layer film, have significantly improved the corrosion resistance of AZ91D. Ti film and Ti-Cr multi-layer film are better than Cr film in words of protection effect. Ti-Cr multi-layer film contributes the best corrosion resistance for magnesium alloy AZ91D.
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